This invention relates to a novel deionization apparatus adapted to transfer ions in a liquid under the influence of a polar field. More specifically, this invention relates to a deionization apparatus adapted to purify aqueous liquids to effect the production of high purity water and the bonding of membranes in such apparatus.
The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture. The most well known processes include electrodialysis, liquid chromatography, membrane filtration and ion exchange. A lesser known methodology is electrodeionization, occasionally mistermed filled cell electrodialysis. Electrodeionization has been difficult to develop as a commercial process due primarily to the inconsistencies of structural design.
In both electrodialysis and electrodeionization, ion exchange membranes are utilized in the apparatus. These membranes have the characteristic of promoting the transfer of the specific ions therethrough under the influence of a polar field. In use, a liquid having a high concentration of undesired ions is passed through a compartment at least one wall of which comprises an ion exchange membrane and, as the liquid passes through the compartment, ions are removed from the compartment through the membrane into an adjacent compartment. The compartment from which the ions are removed is generally known as the diluting compartment while the compartment into which ions migrate are generally termed the concentrating compartment. In electrodeionization apparatus, the dilution compartment is filled with beads of ion exchange resin which promotes the migration of ions into the concentration compartment. In electrodialysis, no resin beads are utilized and the only material within the dilution compartment is the liquid being purified and membrane support screens.
In present electrodialysis and electrodeionization apparatus a plate and frame design is utilized. In one type of device the ion exchange membranes are placed loosely and unbonded in a stack and are held together under pressure. This is undesirable since leaking occurs into the dilution compartments. In addition, in the electrodeionization apparatus it is extremely difficult to construct large devices wherein the spacers, membranes and electrodes are held together without leakage or loss of the resin beads from the dilution compartments. In a second type of device, the membranes are bonded with adhesive, however the bonds are quite weak as the membrane surfaces are usually wet resulting in poor adhesion of the membranes to the support substrate.
In bonding ion exchange membranes, two major problems are encountered. First, the active membranes either must always be wet to prevent damage or they tend to swell after being wet. It is quite difficult to bond a wet membrane and, if a bond is made while dry a dimensional distortion occurs upon subsequent wetting of the membrane which results in a weakening or failure of the bond. Secondly, the active material of the ion exchange membrane is supported on a substrate but is not chemically or heat bonded to the substrate. The bond depends primarily on a mechanical adhesion. Since the active portion of the membrane is usually mechanically weak, and since the membrane and substrate are often incompatible, the bond between the active membrane and the substrate is weak. This results in a bond whose strength is limited by the adhesion strength of the active portion of the membrane to its substrate. Accordingly, it would be highly desirable to provide a means for bonding the ion exchange membrane within an electrodeionization or electrodialysis apparatus which avoids the problems of the prior art. That is, it would be desirable to provide such a means wherein the bonded surface of the membrane need not be wet prior to bonding and the active membrane is fully swollen prior to bonding. Furthermore, it would be desirable to provide such a bonding means wherein the limit of the strength of the bond is not the limit of adhesion between the active portion of an ion exchange membrane and the membrane substrate upon which the active portion is deposited.